Projection display systems employing spatial light modulators, as well as many other image-producing systems, require only information carrying light be projected on the viewing screens so as to obtain desired images of acceptable qualities. Undesired light, especially scattered light from the components of the projection system, if mixed with the information carrying light and projected on the viewing screen will degrade the desired image quality. In particular, undesired scattered light may decrease the contrast ratio.
Contrast ratio is the ratio of luminance between the brightest white that can be produced and the darkest black that can be produced. If a display image has a higher contrast ratio, a viewer will judge it to be sharper than a displayed image with a lower contrast ratio, even if the lower contrast image has substantially more measurable resolution. Contrast ratio can be seriously degraded by light scattered, for example, from the cell glass of the micromirror device and the package that contains the micromirror device. This scattered light typically travels through the projection lens of the display device and is directed on to the target, even when the micromirrors are set for displaying a dark pixel.
Spatial light modulators are key components of such display systems, and may produce significant undesired scattered light. For example, a micromirror-based spatial light modulator comprises an array of reflective and deflectable micromirror devices, which is packaged for protection purposes in handling, delivering, and operation. In operation, incident light of the display system is incident onto the micromirror array, and unavoidably also on the package of the array. Undesired scattered light from the array of micromirrors in the spatial light modulator can be depressed in many ways, as set forth in U.S. patent application Ser. No. 10/305,536, Ser. No. 10/305,536, and Ser. No. 10/305,509 both to Huibers et al. and filed Nov. 26, 2002, the subject matter of each being incorporated herein by reference. Because the package is unavoidably illuminated by the incident light, the package may also produce undesired scattered light that is eventually projected onto the viewing screens, thus resulting in a poor contrast ratio.
Therefore, methods and apparatus for reducing light scattering from a package of a spatial light modulator are needed to improve the display quality.